The Amazingly Agile Boxfish

One look at the aptly named boxfish and you might expect it to swim as well as a barn would fly. But an international team of scientists now finds that water flows over the bony carapaces of these fishes much as air does over the space shuttle, creating vortices of water that help guide the fish while swimming in unpredictable waters. The U.S. Navy hopes these findings will help it design better underwater robots.

Boxfish species dwell in reefs and constantly face turbulence. But despite their unwieldly casings, they are quite dexterous and swim in remarkably stable paths. To delve into the secret of boxfish agility, marine biomechanist Ian Bartol of the University of California, Los Angeles and colleagues had a boxfish known as a smooth trunkfish (Lactophrys triqueter) caught, frozen, and shipped from Puerto Rico.

The team took the fish to UCLA's radiology department for a CAT scan. "It was odd sitting in that waiting room with the fish," Bartol recalls. From the CAT data, the scientists had a three-dimensional, 15-centimeter-long epoxy boxfish model built, which they dunked in a water tunnel seeded with reflective particles. As water flowed over the model, the team illuminated the reflective particles with lasers and videotaped them to track the water's motions.

The vortices of water that develop around the boxfish's body are the secret to its unflappability, the researchers report in the 15 February issue of Journal of Experimental Biology. For instance, if flowing water slants the boxfish upwards, the vortex on top of it becomes strongest right behind its center of mass, sucking its rear end back up and straightening it out. The same phenomenon is a hallmark of delta wing aircraft such as the Concorde or the space shuttle.

"They're actually taking advantage of turbulence, and controlling when and where it appears," says Bob Gisiner, marine mammal science and technology program manager at the Office of Naval Research in Arlington, Virginia. Robots based on this principle would waste less energy and computing power on corrections to their paths, Gisiner says, which might enable them to make smarter, longer missions. Bartol says his team now plans to study other boxfish species to see what tricks they may have hidden in their bony carapaces.